Gas turbine blades are mounted on the circumference of a rotating disc in a circular array as shown in FIG. 1. They are often attached removably to the disc so they can be individually tested, serviced, and replaced. The rotation rate of industrial gas turbines may be 3600 rpm for 60 Hz power generation, and much higher for aero engines. There is aerodynamic stress on turbine blades, but the greatest mechanical stress is the centrifugal force on the blade attachments, which can be 70,000 lbs or more per blade. Herein “centrifugal force” or “reactive centrifugal force” is the force exerted radially outwardly by a body on a structure that retains the body in circular motion.
Each blade includes an airfoil section and a platform that forms an inner shroud ring with adjacent platforms. The inner shroud ring separates the combustion working gas from cooling air supplied to channels in the blade via channels in the disc. Each blade is connected to the disc by an attachment device called a root. In order to distribute the centrifugal loads evenly on opposed surfaces of the root, it is common to align the centers of mass of the airfoil, platform, and root along a rotation radius called an attachment or stacking axis. The goal is actually to have the sum of moments about an attachment plane of the blade to be approximately zero during operation of the blade to balance forces on the blade root lobes. The predominant operating load is the centrifugal load, although the airfoil lift load also contributes to the operating loads to a much lesser degree, so the center of mass of the airfoil and/or platform may be offset by a small dimension from the attachment plane in order to offset the airfoil lift moment.